#include<iostream>
#include<cstring>
#include<vector>
#include<map>
#include<cstdio>
#include<algorithm>
#include<string>
#include<string.h>
#include<queue>
#include<math.h>
#include<set>
#include<functional>
using namespace std;
using ull = unsigned long long;
#define int long long
//using i128 = __int128_t;
inline int inc(int x, int v, int mod) { x += v;return x >= mod ? x - mod : x; }//代替取模+
#define endl '\n'
typedef long long LL;
typedef unsigned long long ULL;
typedef pair<int, int> PII;

const int N = 2e3 + 10, M = 1e5 + 10;

inline int read()
{
    int x = 0, f = 1;
    char ch = getchar();
    while (ch < '0' || ch > '9')
    {
        if (ch == '-')
            f = -1;
        ch = getchar();
    }
    while (ch >= '0' && ch <= '9')
        x = x * 10 + ch - '0', ch = getchar();
    return x * f;
}


void solve()
{
    int n,m;
    cin >> n >> m;
    if(n == m)
    {
        cout << "NO" << endl;
        return;
    }
    if(n - m > 26)
    {
        cout << "NO" << endl;
        return;
    }
    string ans;
    char ss = 'a';
    if(2 * m >= n)
    {
        for(int i = 1; i <= n - m - 1; i ++)
        {
            ans.push_back(ss);
            ss++;
        }
        for(int i = 1; i <= 2 * m - n + 2; i ++)
        {
            ans.push_back(ss);
        }
        ss--;
        for(int i = 1; i <= n - m - 1; i ++)
        {
            ans.push_back(ss);
            ss--;
        }
    }   
    else                                                    
    {
        for(int i = 1; i <= m ; i ++)
        {
            ans.push_back(ss);
            ss++;
        }
        char op = ss - 1;
        for(int i = 1; i <= n - 2 * m; i ++)
        {
            ans.push_back(ss);
            ss++;
        }
        for(int i = 1; i <= m; i ++)
        {
            ans.push_back(op);
            op --;
        }
    }
    cout << "YES" << endl;
    cout << ans << endl;
}
signed main()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    int tt;
    tt = 1;
    cin >> tt;
    while (tt--)
    {
        solve();
    }
}